You may contact Fairfax County police any time at 703-691-2131 and they will enforce the noise ordinance -- no matter what time of day it is. See "...still constitute a noise disturbance under Section 108-5-1 independently of the hours of day said acts take place..." Yes there are references to 11 p.m. - 7 a.m., but you cannot make unreasonably loud noise for the rest of the day or night either.
You can read the law:
Section 108-5-1. - Loud and unnecessary noise.
It shall be unlawful for any person to make, continue to make, or cause to be made or continued, a noise disturbance within the County of Fairfax.
(7-17-68, § 17-4; 24-75-16A; 1961 Code, § 16A.5.1.)
Section 108-5-2. - Specific prohibitions.
The following acts, are declared to be noise disturbances in violation of this Chapter, provided that the acts so specified shall not be deemed to be an exclusive enumeration of those acts which may constitute a noise disturbance under Section 108-5-1, and provided that the acts so specified in Subparagraph (a) below may still constitute a noise disturbance under Section 108-5-1 independently of the hours of day said acts take place.
(a) Operating or permitting the use, or operation of any radio receiving set, musical instrument, television, phonograph, or any other device for the production of sound between the hours of 11 p.m. and 7 a.m. the following day in such a manner as to be plainly audible across property boundaries or through partitions common to two persons within a building or plainly audible at fifty (50) feet from such device when operated within a motor vehicle parked on a public right-of-way or in a public place.
See Section 108-5 of the Fairfax County Code:
http://library.municode.com/index.aspx?clientId=10051
Overview of noise in the County:
http://www.fairfaxcounty.gov/dpz/environment/noise/